Storage of video signals

ABSTRACT

Apparatus for storing a video signal includes a signal compression arrangement constituted by a spatial two-dimensional sub-band filtering arrangement that filters a digital video signal to form data sets constituting respective sub-bands of the two-dimensional spatial frequency domain, a quantizer that quantizes the data sets in accordance with respective values which are such that the amount of quantization of one of the data sets constituting a sub-band to which dc luminance information of the signal is at least predominantly confined is less than the average of the amounts of quantization of the remaining data sets, and an entropy encoder that selectively encodes at least some of the quantized data sets so that the quantized data sets, as selectively entropy encoded, form a compressed video signal. The signal compression arrangement is followed by a storage arrangement for storing the compressed video signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the storage of video signals, and is moreparticularly concerned with apparatus for storing a video signal inwhich the signal is compressed before it is stored.

2. Description of the Prior Art

Compression of video signals on an intra-image basis (for example,compression on an intra-field or intra-frame basis) makes use of theredundancy present in pictures or images represented by the signals toreduce the amount of information needed to represent the pictures orimages. The compression can be used to reduce bandwidth, in the case oftransmission of a video signal, or to reduce storage capacity, in thecase of storage of a video signal.

Intra-image compression can, as is known, be effected in the time domainby the use of differential pulse code modulation, in which a predictoris used to predict the values of samples representing pixels based onprevious pixel values. Since the image pixels are highly correlated, theprediction is accurate and results in a small and uncorrelated error(that is, a difference between the predicted and actual values). Theerror samples are encoded and, since they can be encoded using fewerbits than the samples representing the original pixels, compression canbe achieved.

FIG. 1 of the accompanying drawings shows a known apparatus or systemfor effecting intra-image compression of a video signal in thetwo-dimensional spatial frequency domain. A video signal, which is indigital form and comprises successive multi-bit (for example 8-bit)samples or words each representing a respective pixel of an scannedimage or picture, is applied via an input 10 to a decorrelator 12. Adecorrelated version of the video signal is outputted by thedecorrelator 12 to a quantizer 14 and then to an entropy encoder 16,which together compress the decorrelated version of the video signaloutputted by the decorrelator 12 to produce a compressed signal on anoutput 18. The compressed signal can then be transmitted or stored.(Note that, although the decorrelator 12, quantizer 14 and entropyencoder 16 are shown for clarity as being separate items, they may inpractice be embodied in an at least partially combined form.) Aftertransmission or storage, the compressed signal can be restoredsubstantially to its original form by expansion by way of entropydecoding, dequantizing and correlation operations which employparameters converse to those used for decorrelation, quantization andentropy encoding, respectively, upon compression.

The operation of decorrelation performed in the decorrelator 12 reliesupon the fact that neighboring pixels of an image are highly correlated,whereby processing an image (for example, a field or frame of a videosignal) to form decorrelated signal portions representing differentcomponents of the image in the two-dimensional spatial frequency domainenables a reduction in the amount of information needed to represent theimage. Specifically, the decorrelated signal portions representdifferent spatial frequency components of the image to which the humanpsychovisual system has respective different sensitivities. Thedifferent decorrelated signal portions are subjected to differentdegrees of quantization in the quantizer 14, the degree of quantizationfor each signal portion depending upon the sensitivity of the humanpsychovisual system to the information in that portion. That is, each ofthe decorrelated signals is quantized in accordance with its relativeimportance to the human psychovisual system. This selective quantizationoperation, which is a lossy operation in that it involves deliberatediscarding of some frequency data considered to be redundant or oflittle importance to adequate perception of the image by the humanpsychovisual system, in itself enables some signal compression to beachieved. The quantizer 14 enables compression to be achieved in twoways: it reduces the number of levels to which the data inputted to itcan be assigned, and it increases the probability of runs of zero valuesamples on the data it outputs. Note that, in video signal compressionapparatus described in detail below, the ability to achieve signalcompression provided by the operation of the quantizer 14 is not used toproduce a bit (data) rate reduction in the quantizer itself. Instead, inthat case, the ability to achieve signal compression provided by theoperation of the quantizer is carried into effect in the entropy encoder16 in that the reduction in information content achieved in thequantizer 14 enables a consequential bit (data) rate reduction to beachieved in the entropy encoder.

Further (non-lossy) compression, and bit (data) rate reduction, isprovided in the entropy encoder 16 in which, in known manner, using forexample variable length coding, the data produced by the quantizer 14 isencoded in such a manner that more probable (more frequently occurring)items of data produce shorter output bit sequences than less probable(less frequently occurring) ones. In this regard, the decorrelationoperation has the effect of changing the probability distribution of theoccurrence of any particular signal level, which is substantially thesame as between the different possible levels before decorrelation, intoa form in which in which it is much more probable that certain levelswill occur than others.

The compression/coding system or apparatus as shown in FIG. 1 can beembodied in a variety of ways, using different forms of decorrelation.An increasingly popular form of implementation makes use of so-calledtransform coding, and in particular the form of transform known as thediscrete cosine transform (DCT). (The use of DCT for decorrelation is infact prescribed in a version of the compression system of FIG. 1described in a proposed standard prepared by JPEG (Joint PhotographicExperts Group) and currently under review by the ISO (InternationalStandards Organization).) According to the transform technique ofdecorrelation, the signal is subjected to a liner transform(decorrelation) operation prior to quantization and encoding. Adisadvantage of the transform technique is that, although the wholeimage (for example, a whole field) should be transformed, this isimpractical in view of the amount of data involved. The image (field)thus has to be divided into blocks (for example, of 8×8 samplesrepresenting respective pixels), each of which is transformed. That is,transform coding is complex and can be used on a block-by-block basisonly.

A recently proposed approach to compression/coding in the frequencydomain is that of sub-band coding. In this approach, the decorrelator 12in the system of FIG. 1 would comprise a spatial (two-dimensional)sub-band filtering arrangement (described in fuller detail below) whichdivides the input video signal into a plurality of uncorrelatedsub-bands each containing the spatial frequency content of the image ina respective one of a plurality of areas of a two-dimensional frequencyplane of the image, the sub-bands then being selectively quantized bythe quantizer 14 in accordance with their positions in the sensitivityspectrum of the human psychovisual system. That is, decorrelation isachieved in this case by putting the energy of the overall image intodifferent sub-bands of the two-dimensional spatial frequency domain.Sub-band filtering is believed to provide better decorrelation than thetransform approach. Also, unlike the transform technique, there is norestriction to operation on a block-by-block basis: the sub-bandfiltering can be applied directly to the video signal.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the invention is to provide a video signal recordingapparatus in which the signal is efficiently compressed prior torecordal.

Another object of the invention is to provide a video signal recordingapparatus in which the signal is compressed prior to recordal, and inwhich subjective degradation of picture (image) quality as a result ofcompression and storage (and subsequent reproduction and expansion) isminimized.

A further object of the invention is to provide a video signal recordingapparatus with signal compression in which error correction encoding isapplied to the signal prior to recordal in such a manner thatinformation in the signal of greater subjective importance to the humanpsychovisual system is subjected to a more robust form of errorcorrection encoding.

Yet another object of the invention is to provide a video signalrecording apparatus with signal compression in which error correctionencoding is applied to the signal prior to recordal in such a mannerthat the probability of successful data recovery in shuttle is enhanced.

Still a further object of the invention is to provide a video signalrecording apparatus with signal compression which records a signal suchthat a recognizable picture can be viewed while data is being recoveredin shuttle.

The invention provides apparatus for storing a video signal. Theapparatus comprises signal compression means constituted by a spatialtwo-dimensional sub-band filtering arrangement that filters a digitalvideo signal to form a plurality of data sets constituting respectivesub-bands of the two-dimensional spatial frequency domain, a quantizerthat quantizes the data sets in accordance with respective values, thevalues being such that the amount of quantization of one of the datasets constituting a sub-band to which dc luminance information of thesignal is at least predominantly confined is less than the average ofthe amounts of quantization of the remaining data sets, and an entropyencoder that encodes at least some of the quantized data sets. Thesignal compression means is followed by storage means for storing thecompressed video signal.

In such apparatus, the signal is subjected, before storage, to anefficient and frequency-selective form of compression involving spatialtwo-dimensional sub-band filtering. In this regard, the fact that thedata set constituting the sub-band to which the dc luminance informationof the signal is at least predominantly confined is quantized less thanthe average of the remaining data sets means that there is less loss ofinformation in that part of the two-dimensional spatial frequencyspectrum that is of greatest importance to satisfactory appreciation ofthe image represented by the signal by the human psychovisual system.Thus, subjective degradation of picture (image) quality as a result ofcompression and storage, and subsequent reproduction and expansion, isminimized.

The storage means may be of a variety of forms, for example magnetictape storage means (in which case the apparatus may be in the form of avideo tape recorder with compression), magnetic disk storage means, orrandom access memory storage means (RAM recorder).

Preferably, the apparatus includes error correction coding meansoperative to effect error correction encoding of the signal before it isstored in the storage means. In this way, errors which occur onreproduction, particularly in the case when the storage means is amagnetic tape storage means, can be minimized. Accordingly, degradationof picture quality as a result of compression and storage, andsubsequent reproduction and expansion, is further minimized.

The error correction coding means may be operative to provide differenterror correction encoding as between different portions of the signaleach derived from a respective one or more of said data sets. Thisfeature provides the advantage that different sub-bands may in effect besubjected to different levels or types of error correction as may beconsidered appropriate in accordance with their respective informationcontents. For instance, the error correction coding means may beoperative to provide more robust error correction encoding to a portionof the signal derived from said data set constituting the sub-band towhich the dc luminance information of the signal is at leastpredominantly confined than to at least part of the remainder of thesignal. This feature provides that, in general in the case ofreproduction, the dc luminance sub-band (of most importance to the humanpsychovisual system) is given special attention from the standpoint oferror correction so as even further to minimize degradation of picturequality as a result of compression and storage, and subsequentreproduction and expansion. Further, in the specific case of reproducingand expanding data in a shuttle (high speed reproduction) mode, thisfeature provides the further advantage that the probability ofsuccessful data recovery is enhanced.

As is well known, a color video signal can be in component or compositeform. A component color video signal comprises three separate signalswhich together represent the totality of the video information. Thethree separate signals may, for example, be a luminance signal and twocolor difference signals (Y, Cr, Cb) or three signals each representinga respective color (R, G, B). A composite color video signal, on theother hand, is a single signal comprising all the luminance andchrominance (color) information.

Previously proposed color video signal compression systems as describedabove all operate on component signals only. That is, taking the exampleof the system of FIG. 1, three separate systems as shown in FIG. 1 areneeded, one for each of the three components. Also, if the signal is incomposite form, there is a need for means to convert it into componentform prior to compression. Further, three expansion systems are neededto convert the transmitted or stored signals back to their originalform, together with (if appropriate) means to convert the componentsignals back into composite form. The need to process the video signalin component form thus involves the expense and inconvenience ofconsiderable hardware replication.

While the invention is applicable in the case of component (ormonochrome) video signals, a preferred feature of the invention is thatit can be used also to compress and store composite color video signals.This preferred feature takes advantage of a realization by the inventorsthat, due to the way in which luminance and chrominance information arecombined in conventional broadcast standard (for example, NTSC and PAL)composite color video signals, such a signal can be spatially sub-bandfiltered such that the chrominance information can be (as is explainedin detail below) concentrated in a certain area of the two-dimensionalspatial frequency domain (that is, in certain of the sub-bands),whereby, if the data sets to which the dc chrominance information and dcluminance information are at least predominantly confined are quantizedmore lightly than the other data sets (which contain wholly or largelyonly the ac luminance information) are on average quantized, then sincethe dc information is more important to satisfactory appreciation of theimage by the human psychovisual system than the ac luminance informationit is in fact (surprisingly) possible satisfactorily to compress acomposite color video signal directly, that is without first convertingit to component form and compressing each component individually.

In the case in which, in order to enable compression of a digitalcomposite color signal, the quantizer is operative to quantize said datasets in accordance with respective values which are such that theamounts of quantization of each of the said data sets constituting thesub-bands to which the dc luminance information and the dc chrominanceinformation of the signal is at least predominantly confined are lessthan the average of the amounts of quantization of the other data sets,the error correction coding means is preferably operative to providemore robust error correction encoding to portions of the signal derivedfrom said data set constituting the sub-band to which the dc luminanceinformation of the signal is at least predominantly confined and fromsaid data sets constituting the sub-bands to which the dc chrominanceinformation of the signal is at least predominantly confined than to theremainder of the signal. Thus, there is a relatively smaller probabilityof degradation of these relatively important sub-bands.

The storage means may be operative to record the compressed video signalon a magnetic storage medium, and storage control means may be providedto cause a portion of the compressed signal derived from said data setconstituting the sub-band to which the dc luminance information of thesignal is at least predominantly confined to be recorded on the storagemedium differently than the remainder of the compressed signal. Forexample, in an embodiment disclosed hereinbelow, the storage means isoperative to record the compressed video signal in slanting tracks on amagnetic tape, and storage control means is provided to cause a portionof the compressed signal derived from said data set constituting thesub-band to which the dc luminance information of the signal is at leastpredominantly confined to be recorded on said tracks differently thanthe remainder of the compressed signal. The fact that the data setconstituting the sub-band to which the dc luminance information of thesignal is at least predominantly confined is recorded differently thanthe remainder of the compressed signal data sets leads to the advantagethat the dc sub-band can be so recorded as to improve the probability ofdata recovery in shuttle.

The different recording of the portion of the compressed signal derivedfrom the data set constituting the sub-band to which the dc luminanceinformation of the signal is at least predominantly confined can beeffected in several ways. For instance, the storage control means may beoperative to cause said portion of the compressed signal derived fromsaid data set constituting the sub-band to which the dc luminanceinformation of the signal is at least predominantly confined to berecorded at predetermined positions along (for example at the centersof) at least some of said tracks. If this is done, and if the locus oftravel of a reproducing head used in reproduction is synchronized withrespect to the tracks so that, in shuttle, the locus crosses thepredetermined track positions, at least the dc luminance sub-band, whichcontains enough of the picture information to enable the picture to berecognized, can be recovered in shuttle to enable the tape position tobe monitored during shuttle movement.

Additionally or alternatively, the different recording of the portion ofthe compressed signal derived from the data set constituting thesub-band to which the dc luminance information of the signal is at leastpredominantly confined can be effected in that the storage control meansis operative to cause said portion of the compressed signal derived fromsaid data set constituting the sub-band to which the dc luminanceinformation of the signal is at least predominantly confined to berecorded at least twice.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, and other objects, features and advantages of the inventionwill be apparent from the following detailed description of illustrativeembodiments thereof, which is to be read in conjunction with theaccompanying drawings, in which like references indicate like itemsthroughout, and in which:

FIG. 1 shows a video signal compression apparatus or system forachieving intra-image compression of a video signal in the frequencydomain;

FIG. 2 is a block diagram of one form of implementation of adecorrelator, in the form of a sub-band filtering arrangement, for usein the video signal compression apparatus;

FIG. 3 is a detailed block diagram of a horizontal filter arrangementforming part of the sub-band filtering arrangement shown in FIG. 2;

FIG. 4 shows a sub-band filtered field of a video signal (luminanceonly) on a two-dimensional frequency plane;

FIG. 5 is a block diagram of another form of implementation of adecorrelator, in the form of a sub-band filtering arrangement, for usein the video signal compression apparatus;

FIG. 6 is a graph representing the response of the human psychovisualsystem to different spatial frequencies;

FIG. 7 represents a quantization matrix that would be used in aquantizer of the video signal compression apparatus if a sub-bandfiltered component (luminance) video signal were being processed in thequantizer, and shows also respective modifications to be made if,instead, a sub-band filtered composite video signal (NTSC or PAL) werebeing processed in the quantizer;

FIG. 8 is a block diagram of the quantizer;

FIG. 9 shows part of FIG. 4 on an enlarged scale, and is used to explainthe operation of the quantizer;

FIG. 10 is a diagram showing how zig-zag scanning of the ac sub-bands iscarried out in the quantizer;

FIG. 11 shows the format of quantized data emerging from the quantizerfor ac sub-bands;

FIG. 12 is a block diagram of an entropy encoder forming part of thevideo signal compression apparatus;

FIG. 13 is a representation of the contents of a fixed length codelook-up table forming part of the entropy encoder;

FIG. 14 shows a sub-band filtered field of an NTSC composite color videosignal, sampled at four times its color sub-carrier frequency, on thetwo-dimensional frequency plane;

FIG. 15 is a graph showing the two-dimensional frequency content of afield of an analog NTSC composite color video signal;

FIG. 16 shows a frame of an NTSC composite color video signal, sampledat four times the color sub-carrier frequency, on the two-dimensionalfrequency plane;

FIG. 17 is a view corresponding to FIG. 4, but showing on thetwo-dimensional frequency plane both the sub-band filtered field of anNTSC composite color video signal, and a sub-band filtered field of aPAL composite color video signal, each sampled at four times its colorsub-carrier frequency;

FIG. 18 is a block diagram of a video signal storage apparatus embodyingthe invention;

FIG. 19 is a block diagram of a storage means forming part of theapparatus of FIG. 18; and

FIGS. 20 and 21 show respective examples of how a compressed videosignal may be recorded on a magnetic tape by the storage means of FIG.19.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An apparatus for compressing a digital video signal will now bedescribed with reference to the drawings. The basic construction of theapparatus is in accordance with FIG. 1 (described above). Thedecorrelator 12 of the present apparatus is constituted by a sub-bandfiltering arrangement which, according to one form of implementation asshown in outline form at 12A in FIG. 2, comprises a horizontal filterarrangement 20A, an intermediate field store 22, a transpose sequencer(address generator) 24, a vertical filter arrangement 26A, an outputfield store (FS) 28 and an output sequencer (address generator) 29A. Asexplained above, sub-band filtering can be effected on a separablebasis. Thus, in FIG. 2, filtering in the two orthogonal imagedirections, namely the horizontal direction (the direction of imagescanning in the case of conventional video) and the vertical direction,is effected entirely independently and separately of one another byrespective one-dimensional filtering operations performed in thehorizontal and vertical filter arrangements 20A and 26A, respectively.

The horizontal filter arrangement 20A and vertical filter arrangement26A can be of substantially the same construction as one another. Thus,the construction of the horizontal filter arrangement 20A only will bedescribed in detail.

It will be assumed that the filtering is to achieve 8 sub-bands in eachof the horizontal and vertical directions, that is to say that a squarearray of 64 (8×8) sub-bands is to be produced. It will further beassumed that the 64 sub-bands are (as is preferred) to be of equalextent to one another.

The horizontal filter arrangement 20A is preferably of a tree orhierarchical structure as shown in FIG. 3, comprising three successivefilter stages 30, 32 and 34.

The first stage 30 comprises a low pass filter (LPF) 36 and a high passfilter (HPF) 38, each of which is followed by a respective decimator(DEC) 40. The LPF filter 36, HPF filter 38 and the decimators 40together make up a quadrature mirror filter (QMF). Each of the filters36 and 38 can be a finite impulse response (FIR) filter of conventionalform. In use, a line of a field of the input digital video signal isapplied, sample-by-sample, to the first stage 30, to be low passfiltered and high pass filtered by the LPF 36 and HPF 38, respectively.Thus, the LPF 36 and HPF 38 produce outputs comprising low pass filteredand high pass filtered versions of the input line, respectively, theoutputs representing the spatial frequency content of the line in theupper and lower halves of the horizontal spatial frequency range. Thatis, the first stage 30 divides the input line into two sub-bands in thehorizontal direction. The decimators 40 decimate (sub-sample) therespective outputs by a factor of two, whereby the total number ofsamples outputted by the decimators 40 (together) is the same as thetotal number of samples in the line.

The second stage 32 is of similar construction to the first stage 30,except that there are two QMFs each as in the first stage and the outputfrom each of the decimators 40 of the first stage is passed as an inputto a respective one of the two QMFs. Thus, the second stage 32 producesfour outputs representing the spatial frequency content of the line infour equal quarters of the horizontal spatial frequency range. That is,the second stage 32 further divides the two sub-bands, into which theinput line was divided in the first stage 30, into four sub-bands in thehorizontal direction. The four decimators of the second stage 32decimate (sub-sample) the respective outputs by a factor of two, wherebythe total number of samples outputted by the decimators of the secondstage (together) is the same as the total number of samples in the line.

The third stage 34 is of similar construction to the first stage 30,except that there are four QMFs each as in the first stage and theoutput from each of the four decimators of the second stage 32 is passedas an input to a respective one of the four QMFs. Thus, the third stage34 produces eight outputs representing the spatial frequency content ofthe line in eight equal one-eighths of the horizontal spatial frequencyrange. That is, the third stage 34 divides the four sub-bands into whichthe input line was previously divided into the required eight sub-bandsin the horizontal direction. The eight decimators of the third stage 34decimate (sub-sample) the respective outputs by a factor of two, wherebythe total number of samples outputted by the decimators of the thirdstage (together) is the same as the total number of samples in the line.

The eight outputs of the third stage 34, that is of the horizontalfilter arrangement 20A, are passed to the intermediate field store 22and stored at positions corresponding to respective one-eighths of afirst line thereof. The above process of horizontal filtering is thenrepeated for all the other lines of the field of the input digital videosignal. This results in the intermediate field store 22 containing aversion of the field of the input digital video signal that has beenfiltered into eight sub-bands in the horizontal direction (only). Eachline of the field stored in the intermediate field store 22 is dividedinto eight portions each containing the horizontal spatial frequencyinformation in a respective one of eight sub-bands of the horizontalspatial frequency range of the image that the original fieldrepresented. Thus, the horizontally filtered field stored in theintermediate field store 22 can be considered to be divided into eightcolumns.

Referring back to FIG. 2, the horizontally filtered field stored in theintermediate field store 22 is then fed (under the control of thetranspose sequencer 24) into the vertical filter arrangement 26A, inwhich it is filtered into eight sub-bands in the vertical direction insimilar manner to that in which filtering into eight sub-bands in thehorizontal direction was achieved in the horizontal filter arrangement20A. The horizontally and vertically filtered field is fed on aline-by-line basis into the output field store 28 to be passed fromthere to a quantizer 14A. The store 28 can be considered to have beenpartitioned into an array of 64 (8×8) storage regions, in each of whicha respective one of the 64 sub-bands is stored. Thus, successive fieldsof the input digital video signal are sub-band filtered and passed, dulyfiltered, to the quantizer 14A after a delay of two field intervals.

The transpose sequencer 24 produces read addresses for the intermediatefield store 22, to control reading of the contents thereof into thevertical filter arrangement 26A, as follows. As will be recalled, thesignal as stored in the intermediate field store 22 comprises the linesof the original field, each divided horizontally into eight sub-bands.That is, the signal as stored in the intermediate field store 22 can, asmentioned above, be considered to comprise eight columns. To enable thesignal stored in the intermediate field store 22 to be verticallyfiltered by hardware of the same construction (the vertical filterarrangement 26A) used to horizontally filter it, it must be transposed,that is rotated through 90 degrees, as it is read to the vertical filterarrangement 26A, so that is comprises eight rows (as opposed tocolumns). The transpose sequencer 24 addresses the intermediate fieldstore 22 in such a manner as to accomplish this.

The horizontally and vertically filtered field stored in the outputfield store 28, which has been sub-band filtered by a factor of eight inboth directions, can thus be considered as having been divided intoeight rows and eight columns, that is into an 8×8 sub-band array. Thehorizontally and vertically sub-band filtered field, as stored in theoutput field store 28 of the sub-band filtering arrangement 12 ready forquantization, can be represented (subject to the qualification mentionedbelow concerning sub-band scrambling) on a two-dimensional frequencyplane as shown in FIG. 4. In conventional manner for considering image(two-dimensional) signals, frequency is represented in normalized formin FIG. 4, the symbol pi being equivalent to half the Nyquist limitsampling frequency. For the time being, it is assumed that the inputdigital video signal is a component (luminance) signal, or even amonochrome signal, rather than a composite signal. Thus, the 64sub-bands comprise a single sub-band, referred to hereinafter as the dc(zero spatial frequency) sub-band, which contains most or all of the dcinformation image intensity data, namely the sub-band (shown shaded) inthe upper left hand corner of FIG. 4, together with 63 ac sub-bandswhich contain edge data, that is components of the two-dimensionalfrequency spectrum of the image in respective sub-bands higher than dc(zero spatial frequency). In this regard, if the filtered signal in theoutput field store 28 were viewed on a monitor, it would beintelligible. Thus, a very heavily filtered version of the originalsignal would be seen in the upper left hand corner picture area (dcsub-band) and higher frequency components could be observed in the other63 picture areas (ac sub-bands).

The sub-band filtering arrangement structure described above withreference to FIG. 3 (unlike an alternative arrangement described belowwith reference to FIG. 5), because of its hierarchical QMF structure,"scrambles" the order or sequence of the sub-bands. That is, due to afrequency inversion that takes place in each of the QMFs, if a field ofthe filtered signal in the output field store 28 were viewed on amonitor, there would not be a one-to-one correspondence between thefield as viewed and the showing of FIG. 4. Thus, while the dc sub-bandwould remain in the upper left-hand corner, the frequency planelocations of the 63 ac sub-bands would be different from (that is,scrambled with respect to) their locations in FIG. 4. The locationswould of course be the same for successive fields and can readily bedetermined from the structure of FIG. 3. In other words, while each ofthe 64 storage regions into which the store 28 is partitioned stores arespective one of the 64 sub-bands, the relative positioning of the 63storage regions containing the ac sub-bands is scrambled (in a knownmanner) with respect to the relative positioning of the ac sub-bands asshown in FIG. 4.

In order that the scrambled locations of the 63 ac sub-bands aredescrambled (that is, put into the pattern shown in FIG. 4) before thesub-band filtered signal is passed to the quantizer 14A, the outputsequencer 29A (which can be located, as shown, in the sub-band filteringarrangement 12A, though it could be located elsewhere, for example inthe quantizer 14A), which is connected to the output field store 28 toproduce read addresses therefor to cause the data therein to be read outto the quantizer 14A, is so designed that the data is read out in adescrambled manner, that is in such a manner that the sub-bands assupplied to the quantizer conform to FIG. 4. (The operation of thesequencer 29A in this regard is described in more detail below withreference to FIGS. 9 and 10.)

FIG. 5 shows at 12B a form of implementation of the sub-band filteringarrangement which can be used instead of that (12A) described above withreference to FIGS. 2 and 3. The sub-band filtering arrangement 12Bcomprises a horizontal filter arrangement 20B, a vertical filterarrangement 26B, an output field store 28, and an output sequencer 29B.As in the case of the sub-band filtering arrangement 12A of FIGS. 2 and3, filtering in the horizontal and vertical directions is in this casealso effected entirely separately of one another, namely by respectiveone-dimensional filtering operations performed in the horizontal andvertical filter arrangements 20B and 26B, respectively.

The horizontal filter arrangement 20B is of a conventional FIRstructure, comprising a chain of an appropriate number of one-sampledelay elements 40a, 40b, . . . 40n tapped off to multipliers 42a, 42b, .. . 42n+1 (supplied with respective appropriate weighting coefficientsWC) whose output signals are summed by adders 44a, 44b, . . . 44n; toproduce a horizontally sub-band filtered output signal 45 at the outputof the final adder. Similarly, the vertical filter arrangement 26B is ofa conventional FIR structure, comprising a chain of an appropriatenumber of one-line delay elements 46a, 46b, . . . 46m tapped off tomultipliers 47a, 47b, . . . 47m+1 (supplied with respective appropriateweighting coefficients WC) whose output signals are summed by adders49a, 49b, 49m to produce a horizontally and vertically sub-band filteredoutput signal 48 at the output of the final adder, which signal isstored on a field-by-field basis in the output field store 28. Theoutput sequencer 29B (which can be located, as shown, in the sub-bandfiltering arrangement 12B, though it could be located elsewhere, forexample in the quantizer 14A), is connected to the output field store 28to produce read addresses therefor to cause the data therein to be readout to the quantizer 14A.

It should be noted that the intermediate field store 22 and thetranspose sequencer 24 used in the sub-band filtering arrangement 12A ofFIGS. 2 and 3 are not necessary when the sub-band filtering arrangement12B of FIG. 5 is used. It should however be noted that theabove-described sub-band frequency scrambling that occurs in thesub-band filtering arrangement 12B of FIGS. 2 and 3 also takes place inthe sub-band filtering arrangement 12B of FIG. 5. Thus, the outputsequencer 29B of the sub-band filtering arrangement 12B of FIG. 5 has toperform descrambling.

Before the quantizer 14A is described in more detail, the principle onwhich it operates will be explained with reference to FIGS. 6 and 7.FIG. 6 is a graph representing an empirically determined equationapproximately representing the response of the human psychovisual systemto different spatial frequencies, the vertical axis representing thesensitivity of the human psychovisual system, the horizontal axisrepresenting spatial frequency, and the frequency value fs representingthe Nyquist limit sampling frequency. As can be seen from FIG. 6, thehuman psychovisual system is most sensitive to lower frequencies,peaking at a value just above dc (zero spatial frequency), and thesensitivity rapidly drops as the frequency increases. It is thereforereadily possible for the quantizer 14A to achieve compression of thesub-band filtered video signal by selectively removing information, inconformity with the graph of FIG. 6 (possibly also taking into accountthe amount of aliasing introduced into each sub-band by the sub-bandfiltering), to which the human phychovisual system is effectivelyinsensitive. This is done by quantizing the 64 sub-bands of the sub-bandfiltered video signal by respective appropriate amounts. Specifically,it is assumed that circular symmetry extends the (one-dimensional)response curve of FIG. 6 to two dimensions. (This assumption is believedjustified in that the human psychovisual system is less sensitive todiagonal frequencies than to horizontal and vertical frequencies.) Theresultant generated surface is then integrated under each of the 64sub-band regions to produce an array of 64 numbers (values) which act asthresholds for the purpose of quantization of respective ones of thesub-bands in the quantizer 14A. As will be evident, the numbersdetermine the extent of quantization for their respective sub-bands. If,as in the example described below, the numbers are used to achievequantization by virtue of their being used to divide data arriving fromthe sub-band filtering arrangement 12A or 12B, then the greater thenumber, the greater the quantization threshold and the greater theprobability of a sample in the relevant sub-band having a zero or nearzero value after quantization.

It should be appreciated that the above-described technique ofestablishing the 64 numbers to be used for quantizing the differentsub-bands represents one possible approach only and, even if thisapproach is used, the numbers derived by the somewhat theoretical methoddescribed above may be modified. In more detail, the quality orviewer-acceptability of a picture represented by a video signal whichhas been compressed by the present (or any other) technique andthereafter expanded by a converse technique is, in the final analysis, amatter of subjective opinion. Thus, a final determination of the numbersto used for quantizing the different sub-bands might well best beachieved by selecting rough initial or starting point values by thetheoretical method described above and then refining those initialvalues by viewer testing (trial and error) to produce values judgedsubjectively to be optimum.

The above-described 64 numbers can be stored in the form of aquantization matrix (naturally an 8×8 matrix in the case of an 8×8sub-band filtered signal), for example in a look-up table in aprogrammable read only memory (PROM). FIG. 7 shows an example of an 8×8quantization matrix produced for a particular design of sub-bandfiltering arrangement. The positioning of the numbers in the matrix ofFIG. 7 corresponds to the positioning of the sub-bands in FIG. 4. Thatis, for example, the number 68 applies to the dc sub-band and the number8192 applies to the ac sub-band in the bottom right-hand corner in FIG.4. It will be seen that the dc sub-band is only lightly quantized(number=68). Although the two ac sub-bands horizontally and verticallyadjacent to the dc sub-band are quantized marginally even more lightlythan the dc sub-band (number=64), the amount of quantization(quantization threshold) of the dc sub-band is, as can clearly be seenfrom FIG. 7, considerably less than the average of the amounts ofquantization (quantization thresholds) of the ac sub-bands.

The following two factors must be borne in mind concerning thequantization matrix.

(a) The relative values of the numbers, rather than their absolutevalues, are of importance. In this regard, as explained below, thenumbers in the quantization matrix may be scaled before they are used toeffect quantization of the sub-bands in the quantizer 14A.

(b) Since, as mentioned above in the description of FIG. 4, it is beingassumed for the time being that the input digital video signal is acomponent (luminance) signal, rather than a composite signal, thenumbers represented in FIG. 7 apply to a component (luminance) signal.(The modifications made to the quantization matrix of FIG. 7 in the caseof processing a composite signal are explained below.)

In the light of the foregoing explanation of its principle of operation,the quantizer 14A will now be described with reference to FIGS. 8 to 11.

FIG. 8 shows the quantizer 14A in block diagram form. The quantizer 14Acomprises a divider 50 that receives data read thereto from the outputfield store 28 of the sub-band filtering arrangement 12A or 12B underthe control of the output sequencer 29A or 29B, and outputs quantizeddata from the quantizer 14A to the entropy encoder 16 (FIG. 1).

The above-mentioned quantization matrix, referenced 52 in FIG. 8, andstored for example in a look-up table in a PROM, is connected to oneinput of a multiplier 54. A scale factor generator 56 is connected toanother input of the multiplier 54. A sequencer (address generator) 58is connected to the quantization matrix 52 to control it so that itoutputs the appropriate one of the 64 numbers stored in the matrix atthe correct time, that is so that each sample supplied to the quantizeris quantized in accordance with the sub-band in which it is located, andis connected to the entropy encoder 16A to supply thereto a timingsignal that indicates to the entropy encoder whether data being suppliedby the quantizer 14A to the entropy encoder results from quantization ofthe dc sub-band or quantization of the ac sub-bands.

The scale factor generator 56 multiplies each of the 64 numbersoutputted by the quantization matrix 52 by a scale factor, whereby thesamples of the stored field supplied to the quantizer 14A are divided inthe divider 50 by the product of the scale factor and the numbercurrently outputted by the quantization matrix 52. The scale factor isusually kept constant throughout the period during which the same storedfield is supplied to the quantizer 14A from the sub-band filteringarrangement 12A or 12B, whereby the values for the different sub-bandsamples as applied by the multiplier 54 to the divider 50 maintain thesame relationship relative to one another over the field as do thenumbers (shown in FIG. 7) in the quantization matrix 52. However, theabsolute values applied by the multiplier 54 to the divider 50 aredetermined by the value of the scale factor. Variation of the scalefactor therefore can vary the output data (bit) rate of the entropyencoder 16A, that is of the entire compression apparatus, and cantherefore be employed, for example, to keep the data rate (which canvary with image content) constant.

The quantizer 14A reads and processes a field of data stored in theoutput field store 28 of the sub-band filtering arrangement 12A or 12B,and passes it on after processing to the entropy encoder 16A. Theprocessing comprises, as explained above, and as described in moredetail below, a selective quantization operation used to achievecompression of the video signal. In addition, as explained below, theprocessing involves arrangement of the data outputted to the entropyencoder in a format that readies it for entropy encoding and bit ratereduction.

Since, in the quantizer 14A described above with reference to FIG. 8,the quantization is effected by dividing the input data (in the divider50), the numbers (FIG. 7) in the quantization matrix 52 must be suchthat those for sub-bands that are to be quantized by a relatively largeamount are greater than those for sub-bands that are to be quantized bya relatively small amount. Instead, the quantization could be effectedby multiplying the input data (in a multiplier taking the place of thedivider 50), in which case the numbers in the quantization matrix 52would be such that those for sub-bands that are to be quantized by arelatively large amount are smaller than those for sub-bands that are tobe quantized by a relatively small amount. (For example, in the lattercase the numbers in the quantization matrix 52 could be reciprocals ofthose shown in FIG. 7.) It will be appreciated that, in both cases, theamount of quantization of the dc sub-band is considerably less than theaverage of the amounts of quantization of the ac sub-bands.

FIG. 9 shows a part (the upper left-hand corner) of FIG. 4 on anenlarged scale. More accurately, FIG. 9 is a map of a sub-band filteredfield as supplied to the quantizer 14A from the output field store 28 ofthe sub-band filtering arrangement 12A or 12B, each sub-band beingstored (as mentioned above) in a respective one of an 8×8 array ofregions into which the store 28 can be considered to be partitioned. Inthis regard, the stored field comprises an 8×8 array of sub-bandsfiltered from the corresponding field of the input video signal.

A field of, for example, an NTSC digital video signal has a horizontalextent of 910 samples and a vertical extent of 262 samples. The sub-bandfiltering described above is however carried out on the active part onlyof the field, which-part comprises 768 samples in the horizontaldirection and 248 samples in the vertical direction. (In fact, there are243 active samples, corresponding to the number of active lines, in theactive part of an NTSC field. In order to produce numbers of activesamples in both directions that are integrally divisible by 8, 5 blanklines are added to make the number of active samples in the verticaldirection equal to 248.) Thus, each of the 64 sub-band areas in theactive sub-band filtered field comprises (768/8)×(248/8)=2976 samples,that is an array of 96×31 samples (as shown in FIG. 9). (The wholeactive field comprises, of course, 64 times that number of samples.) Theoutput sequencer 29A or 29B of the sub-band filtering arrangement 12A or12B is operative to output the samples of the active field stored in theoutput field store 28 of the sub-band filtering arrangement 12A or 12Bas follows.

The sequencer 29A or 29B first causes all of the 2976 samples formingthe dc sub-band (the upper left-hand sub-band area in FIG. 9), namelythose in that one of the 64 regions of the output store 28 of thesub-band filtering arrangement 12A or 12B containing the dataconstituting that sub-band, to be fed in turn to the quantizer 14A. Thiscan be done by addressing the relevant regions of the output store 28 inan order akin to the raster scan employed to form the full active field,though in this case the area (and the number of samples) is reduced by afactor of 64 as compared to a full field. The process is representedschematically by the arrowed lines drawn in the upper left-hand sub-bandarea in FIG. 9. The resulting 2976 samples are supplied in turn to thedivider 50. While this process is taking place, the sequencer 58 (which,though shown as a separate item, could be combined with the outputsequencer 29A or 29B of the sub-band filtering arrangement 12A or 12B)causes the quantization matrix 52 to output to the multiplier 54 thenumber (68) for the dc sub-band. Thus, all the 2976 samples of the dcsub-band are quantized (by the same amount) by being divided in thedivider 50 by the product of the number (68) for the dc sub-band and thescale factor (from the scale factor generator 56), and passed on as arun or sequence of 2976 samples to the entropy encoder 16A. Also, whilethe above process is taking place, the sequencer 58 causes the timingsignal that it supplies to the entropy encoder 16A to be such as toindicate to the entropy encoder that the quantized samples that it isreceiving relate to the dc sub-band.

When the dc sub-band samples have been processed through the quantizer14A as just described, the sequencer 58 causes the timing signal that itsupplies to the entropy encoder 16A to be such as to indicate to theentropy encoder that the quantized samples that it is about to receiverelate to the ac sub-bands. Thus, the timing signal is changed once perfield; that is, it has a frequency equal to the field frequency. Theoutput sequencer 29A or 29B then causes writing to the quantizer 14A ofthe ac sub-band data, and the sequencer 58 causes a correspondingselection of the numbers to be outputted by the quantization matrix 52,in a manner now to be described.

The ac sub-band data is processed through the quantizer 14A in a ratherdifferent manner than the dc sub-band data. An operation is carried out2976 times, under the control of the output sequencer 29A or 29B, ineach of which the respective 63 samples having a respective one of the2976 spatial positions (pixel sites) in the 63 sub-bands are passed tothe divider and multiplied by their respective coefficients. Thisoperation may be more readily understood by referring to FIG. 9.

In the first of the above-mentioned 2976 operations, as a first step thefirst stored sample accessed is the top left-hand one (indicated by adot) in the ac sub-band numbered 1 in FIG. 9. That sample is divided bythe product of the scale factor and the number in the quantizationmatrix 52 relating to that sub-band, that is the number 64: see FIG. 7.Next, as a second step, the same process is repeated for the topleft-hand sample (again indicated by a dot) in the ac sub-band numbered2 in FIG. 9, the number outputted by the quantization matrix 52 in thiscase being the number 64. As a third step, the process is repeated forthe ac sub-band numbered 3 in FIG. 9, the number outputted by thequantization matrix 52 in this case being the number 84. The process isrepeated until it has been carried out 63 times, that is for all of the63 ac sub-bands. The order in which the sub-bands are accessed is inaccordance with the sequence 1 to 63 in which the ac sub-bands aredesignated in FIG. 10 (and, for some only of the ac sub-bands, in FIG.9). It will be seen from FIG. 10 that the order of processing orscanning of the ac sub-bands is a zig-zag order (shown partially byarrowed chain-dotted lines in FIG. 9 for the top left-hand samples) inthat it involves scanning the ac sub-bands in a diagonal direction andin opposite senses. (Thus, the legs of the zig-zag comprise successiveones of a series of groups of the 63 ac sub-bands in a sequence asbetween the groups (legs of the zig-zag) of ac luminance information ofincreasing spatial frequency.) The above-explained zig-zag scanningtechnique is based upon, though considerably modified with respect to, azig-zag scanning technique (described below) that has been proposed aspart of the above-mentioned JPEG (Joint Photographic Experts Group)standard, which (rather than sub-band filtering) requires the use of DCTcoding with 8×8 sample blocks, to each of which an 8×8 DCT transform isapplied, as mentioned at the beginning of this description.

The remaining ones of the above-mentioned 2976 (63-step) operations arecarried out in the same manner as the first one, except that, in eachcase, a respective different one of the 2976 sample sites is used. Thus,for example, in the second operation the samples that are processed arethose having the spatial positions indicated by crosses in FIG. 9, thesebeing those immediately to the right of those, indicated by dots, thatwere processed in the first of the operations.

It will be understood from the foregoing explanation that the datainputted to and outputted by the quantizer 14A for the ac sub-bands(only) has a format as represented in FIG. 11. That is, 2976 successiveseries (hereinafter referred to as "scans")--represented in FIG. 11 byhorizontal strips--of 63 quantized samples are sent to the entropyencoder 16, each such scan relating to a respective one of the 2976sub-band pixel sites and each such scan having employed the zig-zagtechnique of scanning the 63 ac sub-bands as described above. The totalnumber of samples sent to the entropy encoder 16A per field (includingthe dc sub-band and the ac sub-bands) is the same as the number ofsamples in the stored sub-band filtered field written to the quantizer.However, as will be evident from the foregoing explanation, the datasent to the entropy encoder no longer has any resemblance to a videofield.

During the writing of the dc and ac data from the field store 28 to thequantizer 14A under the control of the sequencer 29A or 29B, thesequencer 58 is operative to control the quantization matrix 52 suchthat each sample supplied to the quantizer is appropriately quantized.Specifically, the matrix 52 first continuously outputs the number (68)for the dc sub-band for a period having a duration of 2976 samples, andthen outputs the 63 numbers for the ac sub-bands in a 63-stagesample-by-sample zig-zag manner corresponding to the manner in which thesamples are written from the field store 28 to the quantizer 14A.

The aim of reducing information in the video field by the quantizingoperation performed in the quantizer 14A, and therefore enablingcompression to be achieved by virtue of the quantizing operation, isachieved by the division operation performed in the divider 50. Thus,particularly for the higher frequency sub-bands, and particularly forimage positions that contain little ac spatial frequency information,the sample outputted by the divider 50 will have a zero or very lowvalue, being constituted wholly or mostly by bits of the value zero. Itshould, however, be noted that, at least in the apparatus presentlybeing described, no reduction in bit (data) rate is carried out in thequantizer 14A. That is, the bit length of each sample outputted by thedivider 50 is the same as that of the sample inputted to it. However,the presence of long runs of zero value samples in the data outputted bythe quantizer 14A, and the reduction in the number of levels to whichthe data inputted thereto can be assigned, enables a consequential bitrate reduction to be effected in the entropy encoder, as describedbelow.

The entropy encoder 16A of the video signal compression apparatus may beembodied in the form shown in FIG. 12. The entropy encoder 16A shown inFIG. 12 complies with a so-called "baseline" version of theabove-mentioned JPEG standard, which version sets out minimalrequirements for complying with the standard, whereby it is in manyrespects of known form or based on known technology and will thereforenot be described in great detail.

The entropy encoder 16A shown in FIG. 12 comprises a switch 60controlled by the above-mentioned timing signal provided to the entropyencoder 16A by the sequencer 58 (FIG. 8) of the quantizer 14A. When thetiming signal indicates that the data emerging from the quantizer 14Arelates to the ac sub-bands, that is when such data is one of the 2976successive scans (each having a length of 63 samples) represented inFIG. 11, the switch 60 directs the data to a run length detector/datamodeller 62. When, on the other hand, the timing signal indicates thatthe data emerging from the quantizer 14A relates to the dc sub-band,that is when such data is the run or sequence of 2976 samples of the dcsub-band preceding the 2976 successive scans represented in FIG. 11, theswitch 60 directs the data to a differential pulse code modulator (DPCM)64. The switch 60 is thus changed over once per field.

The detector/modeller 62 is connected to a PROM 66 containing a variablelength code (VLC) look-up table and to a PROM 68 containing a fixedlength code (FLC) look-up table. An output of the detector/modeller 62is connected via a multiplexer 70 to the output 18 of the apparatus.

An output of the DPCM 64 is connected to a data modeller 72, an outputof which is in turn connected via the multiplexer 70 to the output 18 ofthe apparatus. In similar manner to the detector/modeller 62, themodeller 72 is connected to a PROM 74 containing a VLC look-up table andto a PROM 76 containing an FLC look-up table. The VLC PROMs shown at 66and 74 may in fact be the same PROM: they are shown as being separate inFIG. 12 largely for the sake of clarity. Similarly the FLC PROMs shownat 68 and 76 may in fact be the same PROM. Further, rather than being(as shown) a separate item, the modeller 72 can be a part (sub-set) ofthe detector/modeller 62.

The operation of the entropy encoder 16A shown in FIG. 12 will now bedescribed, considering first the case in which the data arriving fromthe quantizer 14A relates to the ac sub-bands and is therefore directedby the switch 60 to the detector/modeller 62.

The detector/modeller 62 examines each of the 2976 63-sample scans (FIG.11) arriving from the quantizer 14A and looks for runs of consecutivezero value samples each preceded and followed by a sample of non-zerovalue. The detector/modeller 62 models the incoming data by convertingeach such run of zero consecutive value samples to a word pair of thefollowing form:

    [RUNLENGTH,SIZE][AMPLITUDE].

The two components or "nibbles" (RUNLENGTH and SIZE) of the first wordof the pair each have a length of 4 bits. The bit pattern of the firstnibble (RUNLENGTH) represents in binary form the number of consecutivezero value samples in the run and is generated by a counter (not shown)that counts the number of consecutive zero value samples following aprevious non-zero value. (Run lengths from 0 to 15 are allowed and arunlength continuation is indicated by a code [F,0].) The bit pattern ofthe second nibble (SIZE) represents the number of bits to be used toindicate the amplitude of the sample of non-zero (value) amplitude thatfollows the consecutive run of zero value samples and is looked up fromthe table--represented in FIG. 13--contained in the FLC PROM 68, theleft hand part of FIG. 13 representing ranges of actual values (indecimal form) and the right hand part representing values of SIZE forthe different ranges. The second word (AMPLITUDE) of the pair representsthe amplitude of the sample of non-zero value in the form of a number ofbits determined by the value of SIZE. For a positive non-zero value,AMPLITUDE is the result of truncating the non-zero value (in binaryform) to have only the number of bits specified by SIZE. For a negativenon-zero value, the non-zero value is decremented by one and the sametruncation procedure is followed. To illustrate the nature of the wordpair by way of an example, suppose that the detector/modeller 62 detectsa run of 4 samples of zero value followed by a sample having a value(amplitude) of +7. In this case, the word pair will be as follows:

    [4,3][111].

The number 4 (or, more accurately, its binary equivalent, namely 0100)for RUNLENGTH indicates that the length of the run of zero value samplesis 4. The number 3 (or, more accurately, its binary equivalent, namely0011) for SIZE indicates (as can be seen from FIG. 13) that 3 bits areused to represent the number +7, namely the amplitude (in decimal form)of the sample of non-zero value (amplitude). The number 111 is in factthe amplitude (+7) of the sample of non-zero value expressed in binaryform and truncated to 3 bits.

It will be appreciated that the above operation will be carried out forthe whole of each scan and that a sequence of word pairs will begenerated for each scan. The number of word pairs (that is, the lengthof the sequence of word pairs) generated for each scan will depend uponthe picture content. In general, the greater the number and length ofruns of zero value samples, the lesser the number of word pairs.

The operation of the detector/modeller 62 as so far described representsonly the first of two stages of data (bit) rate reduction carried out inthe detector/modeller. This first stage represents a reduction in bitrate resulting from the above-described reduction of informationeffected in the quantizer 14A that results (without perceptibledegradation in picture content) in a large number of samples of zerovalue (and, more especially, runs thereof) emerging from the quantizer,especially in the data relating to the ac sub-bands.

The second stage of data rate reduction effected in thedetector/modeller 62 is achieved as follows. The first of each of theabove-mentioned word pairs is replaced in the data outputted from thedetector/modeller 62 with a code therefor looked up in the VLC PROM 66.The VLC PROM 66 stores a respective such code for each possible value ofthe first word. The codes are of different lengths, and their lengthsare selected such that the length of each code is, at leastapproximately, inversely proportional to the probability of theassociated word value occurring. In this way, a further reduction in thedata (bit) rate, resulting from entirely loss-free compression, isachieved.

The operation of the entropy encoder 16A shown in FIG. 12 will now bedescribed for the case in which the data arriving from the quantizer 14Arelates to the dc sub-band and is therefore directed by the switch 60 tothe DPCM 64. The dc sub-band (unlike the ac sub-bands) is subjected toDPCM treatment. Since the dc sub-band contains the intensity informationof the original image (field), it has similar statistics to the originalimage. The ac sub-bands, on the other hand, contain sparse image edgeinformation separated by zero value data and thus have completelydifferent statistics to the dc sub-band. Consequently, it is believeddesirable to entropy encode the ac and dc sub-band data separately andin respective different manners to minimize the overall data rate.

Specifically, the dc sub-band data is treated, firstly, in the DPCM 64,prior to entropy encoding proper. The DPCM 64 uses a previous samplepredictor with no quantization of the error data, because the fact thatthe dc sub-band data represents only a small proportion of the overalldata means that high complexity DPCM treatment is difficult to justify.The DPCM 64 decorrelates (adjusts the probability distribution of) thedc sub-band samples so that a greater degree of compression can beachieved in the modeller 72.

Next, entropy encoding proper, resulting in a reduction in the datarate, is carried out in the data modeller 72. The modeller 72 operatessimilarly to the detector/modeller 62, except that there is no detectionof runs of zero value samples, such runs being much less likely in thedc sub-band.

The modeller 72 models the incoming data by converting the incoming datato a sequence of word pairs of the following form:

    [SIZE][AMPLITUDE].

As in the case of the ac sub-band data, SIZE is looked up from the FLCtable of FIG. 13 (in the FLC PROM 76) and indicates the number of bitsused to represent AMPLITUDE. The bits used to represent AMPLITUDE aredetermined in the same way (truncation) as in the case of ac sub-banddata. The word SIZE is then encoded in that it is replaced in the dataoutputted from the modeller 72 with a code therefor looked up in the VLCPROM 74. The VLC PROM 74 stores a respective such code for each possiblevalue of the word. The codes are of different lengths, and their lengthsare selected such that the length of each code is, at leastapproximately, inversely proportional to the probability of theassociated word value occurring. In this way, a further reduction in thedata (bit) rate, resulting from entirely loss-free compression, isachieved.

FIG. 14 is a graph, corresponding to FIG. 4, showing, on thetwo-dimensional frequency plane, what the inventors have discoveredhappens when a field of a digital NTSC composite video signal, sampledat a frequency equal to four times the color sub-carrier frequency fsc(fsc is approximately equal to 3.58 MHz), is sub-band filtered in avideo signal compression apparatus as described above. The dc and acluminance data is distributed among the 64 sub-bands in substantiallythe same way as described above for a component (luminance) signal.Surprisingly, however, it was found that the chrominance data, or atleast the chrominance data that is needed, is largely (substantially)restricted to two only of the sub-bands (shown shaded in FIG. 14),namely to those two adjacent sub-bands (hereinafter referred to as "dcchrominance sub-bands") at the bottom center in FIG. 14. Attempts havebeen made on an ex post facto basis to explain this phenomenon.

As regards the horizontal positioning of the dc chrominance information,this seems on consideration to be appropriate since it should becentered around the position pi/2 along the horizontal axis of FIG. 14by virtue of the use of a sampling frequency equal to 4.fsc. Thus, if asampling frequency of other than 4.fsc were used, the dc chrominanceinformation would be displaced horizontally from the position shown inFIG. 14. If this were the case, the horizontal positioning of thesub-bands to be treated as the dc chrominance sub-bands would differfrom that described above.

As regards the vertical positioning of the dc chrominance information inFIG. 14, this can be explained as follows. FIG. 15 is a graph showingthe two-dimensional frequency content of a field of an analog NTSCcomposite color video signal, the horizontal axis being in units of MHzand the vertical axis being in units of cycles per picture height (cph).It is of course known that analog NTSC is characterized by a luminancebandwidth of 5.5 MHz and a chrominance bandwidth of 1.3 MHz modulatedabout the color sub-carrier frequency of 3.58 MHz. It is also known thatthe number of sub-carrier cycles per line is 227.5, as a result of whichthe phase of the sub-carrier is shifted by 180 degrees for each line.This is responsible for a modulation of the chrominance signalvertically, which, as shown in FIG. 15, leads to the chrominance beingcentered at a spectral position of 131.25 cph. This appears to explainthe vertical positioning of the chrominance information in FIG. 14.Thus, the process of modulation generates lower and upper sidebands.Since the vertical carrier frequency is at the Nyquist limit frequency,the upper sidebands are on the other side of the Nyquist limit and thusdo not form part of the frequency plane of FIG. 14. Therefore, for NTSC,the dc chrominance data will appear at the bottom of FIG. 14.

As regards the horizontal extent of the dc chrominance information, thefairly harsh filtering (horizontal bandwidth restriction) to which thecolor (chrominance) information is subjected before it is modulated ontothe luminance information appears to explain why the horizontal extentof the chrominance is restricted as shown in FIG. 14, namely so that itfalls largely within two horizontally adjacent ones of the 64 sub-bandsemployed in this case, that is so that the horizontal extent is equal toabout pi/4. (In fact, as explained below, the dc chrominance data infact "spills over" somewhat into the two sub-bands in the bottom row ofFIG. 14 that are horizontally adjacent to those shown shaded.)

It seems on reflection that the vertical extent of the needed colorinformation in FIG. 14 is restricted to about the height of one of thesub-bands, namely about pi/8, for the following reason. It is probablethat the dc chrominance information is wholly or largely restricted tothe two sub-bands shown shaded at the bottom of FIG. 14. It is likewiseprobable that ac chrominance appears in at least some of those sub-bandsabove the two shown shaded at the bottom of FIG. 14. However, since thehuman psychovisual system has a low sensitivity to high frequency (ac)chrominance information, it appears to produce subjectively acceptableresults if any such sub-bands that are co-occupied by ac luminance andac chrominance information are treated as if they are occupied only byac luminance information.

However, whatever the explanation, the restricted bandwidth (in bothdirections) of the needed color information has proven very fortunatebecause, as is explained below, it leads to the advantageous effectthat, with very minor modification, the apparatus as described above canhandle an NTSC composite color video signal. Thus, conversion of thesignal to component form, and tripling of the hardware to handle thethree components separately, is not necessary, leading to a large savingin expense.

The only modification that has to be made to the apparatus as describedabove to enable it to handle an NTSC color composite signal is to changethe numbers in the quantization matrix 52 that determine the amount ofquantization of the sub-bands that contain the dc chrominance data,namely the two dc chrominance sub-bands as shown shaded in FIG. 14.Specifically, instead of being heavily quantized as high frequency acluminance sub-bands of relatively little importance, the two sub-bandsshould be relatively lightly quantized so as to preserve the dcchrominance information. The amount of quantization is in fact desirablyreduced to about the same level as applied to the dc luminance sub-band.The necessary effect can therefore be achieved by changing the twobottom center numbers in the quantization matrix as represented in FIG.7 from their values of 1856 and 2491, for a component (luminance)signal, to 68 (or thereabouts) for an NTSC composite signal. This isshown schematically in FIG. 7.

In principle, no changes other than the above-described change to twonumbers in the quantization matrix 52 are necessary to enable theapparatus to handle a digital NTSC composite color video signal. Inparticular, it is to be noted that the (now lightly quantized) dcchrominance sub-bands can be handled in the quantizer 14A and entropyencoder 16A together with, and in the same manner as, the ac luminancesub-bands.

Although, in principle, only the above-described change in thequantization is necessary to enable the apparatus to handle a digitalNTSC color composite signal, another change that can advantageously bemade is as follows. The zig-zag sequence or order in which, for acomponent (luminance) signal, the 63 sub-bands other than the dcluminance sub-band are quantized and then entropy encoded is, asexplained above, shown in FIG. 10. It will be seen that, in the case ofa digital NTSC color composite signal, the dc chrominance sub-bands havethe positions 49 and 57 in the sequence. This could result in a decreasein the efficiency of compression in that the dc chrominance sub-bandsare much more likely than the adjacent sub-bands in the sequence tocontain non-zero value samples: that is, they could break up runs ofzero value samples. (This is even more likely in the case of PAL thanNTSC because, as explained below, in the case of PAL there are four dcchrominance sub-bands positioned in the center of the frequency plane asshown in FIG. 14.) Thus, preferably, the apparatus is further modifiedin that the sequencer 29A (or 29B) is modified to change the zig-zagsequence so that the dc chrominance sub-bands occupy (in any specifiedorder) the first positions in the sequence and the remaining sub-bandsoccupy the remaining positions in the sequence in the same order asbefore. That is, in the case of an NTSC signal, and using the samenumbering system for the sub-bands as shown in FIG. 10, the sequencewill comprise, in the following order, sub-band 49 (or 57), sub-band 57(or 49), sub-bands 1 to 48, sub-bands 50 to 56, and sub-bands 58 to 63.(The changed sequence that would be adopted in the case of a PAL signal,as will be clear from the description given below with reference to FIG.17, will be sub-bands 24, 31, 32 and 39 (in any order), sub-bands 1 to23, sub-bands 25 to 30, sub-bands 33 to 38, and sub-bands 40 to 63.) Thesequencer 58 in the quantizer 14A (if separate from the sequencer 29A or29B) is modified in correspondence with the way in which the sequencer29A or 29B is modified in order to ensure that each sub-band isappropriately quantized. That is, instead of outputting the 63 numbersfor the sub-bands other than the dc luminance sub-band as shown in FIG.7 in the same zig-zag order as that in which the sub-bands other thanthe dc luminance sub-band are numbered 1 to 63 in FIG. 10, the sequencer58 is modified so that it outputs those numbers in an order which ismodified in the same way in which the zig-zag sequence of quantizing thesub-band filtered samples is (as was just explained) modified.

Further consideration was given to the phenomenon of spectralconcentration of the color information by examining the two-dimensionalfrequency plane for a frame (as opposed to a field) of a digital NTSCcomposite color video signal sampled at 4.fsc, as shown in FIG. 16. Itwill be seen that the composite data in the center of the frequencyplane is composed of four distinct regions due to modulation of thenegative frequencies. These four regions are identical except forfrequency inversion and a phase shift. Ideally, as explained below, thechrominance data should be restricted to a small number of thesub-bands. FIG. 16 indicates that the use of 64 (8×8) sub-bands is agood choice in this respect.

Ideally, the horizontal extent or span of the sub-bands should equal thebaseband chrominance bandwidth for efficient compression. This isbecause, in this case, the chrominance information falls exactly withinthe relevant sub-bands, that is it occupies the whole of those sub-bandsand does not occupy parts of adjacent sub-bands, so that all of the dcchrominance information is lightly quantized and no substantial amountof adjacent ac luminance information is lightly quantized. In otherwords, a smaller span would lead to the chrominance data falling into agreater number of sub-bands (which is in conflict with theabove-mentioned requirement of keeping the number of chrominancesub-bands as small as possible) and a greater span would lead to theadjacent luminance data not being appropriately quantized.

It will be seen from FIG. 16 that there is in fact a small overlap or"spill over" of chrominance data into adjacent sub-bands which aretreated as ac luminance sub-bands, whereby the overlapping parts of thechrominance will be (heavily) quantized in accordance with thequantization thresholds set for those adjacent sub-bands. In practice,it is believed that the results will nonetheless be subjectivelyacceptable. The overlap occurs in the horizontal direction because, ascan be seen from FIG. 16, the horizontal extent of each sub-band isapproximately equal to 0.9 MHz, whereas the chrominance data has abandwidth (two sidebands) of 1.3 MHz, which is slightly larger.Provided, of course, that the overlap is not so large that a significantamount of low-frequency chrominance information spills over intoadjacent sub-bands which are treated in the quantization process as acluminance sub-bands, the overlap will generally be tolerable because, asexplained above, it will comprise higher frequency chrominanceinformation to which the human psychovisual system is not verysensitive. However, the overlap could be avoided, in theory, by slightlyincreasing the size of the sub-bands in either or both directions, thatis by slightly decreasing the total number of sub-bands. Thus, aninspection of FIG. 16 indicates that the overlap would be reduced if a7×7 or a 6×6 array were used. While such an array is realizable intheory, it could not be realized in the case of the "tree" or"hierarchical" QMF structure described with reference to FIGS. 2 and 3because this can only produce, in each direction, a number of sub-bandswhich is an integral power of two. Thus, if the tree structure is to beused, the overlap described above could be avoided only by going down toa 4×4 array. While a 4×4 array is usable and produces acceptableresults, it would result in the extent of the sub-bands that would haveto be used as chrominance sub-bands (which, similarly to FIG. 14, wouldbe the two at the bottom center of the 4×4 array) being substantiallygreater than the extent of the dc chrominance data. Also, it wouldreduce the efficiency of compression by virture of the fact that thenumber of sub-bands would be greatly reduced. The reason for this is asfollows.

The amount of compression achievable by virtue of the quantization stepdecreases, up to a certain extent, as the number of sub-bands decreases.This is because the ratio between the number of ac luminance sub-bandsand the number of dc (luminance and chrominance) sub-bands will increasewith the total number of sub-bands and the ac sub-bands are on averagemore heavily quantized than the dc sub-bands. Thus, for example, inabove-described case in which there are 64 sub-bands, of which one is adc luminance sub-band and two (for NTSC)--or four (for PAL, seebelow)--are dc chrominance sub-bands, either 61 (for NTSC)--or 59 (forPAL)--of the 64 sub-bands are ac luminance sub-bands. That is, either61/64 or 59/64 of a field can be relatively heavily quantized onaverage, thereby enabling a higher degree of compression to be achievedthan would be the case if the number of sub-bands were less than 64.(Thus, for example, if 16 (4×4) sub-bands were used, only 13/16 of afield (for NTSC) would be ac luminance sub-bands.) Therefore, it is ingeneral desirable to use as large a number of sub-bands as is practical,bearing in mind, however, that hardware realization will becomeimpractical if too many sub-bands are used. Also, if a large increase(over an 8×8 array) is made in the number of sub-bands, there will be nonet benefit (or at least not a greatly increased benefit) because morethan two of the sub-bands (for NTSC) or more than four of the sub-bands(for PAL) may have to be treated (due to extensive overspill ofchrominance information) as dc chrominance sub-bands. At present, theuse of an 8×8 square array (or a non-square array of similar size) isbelieved to provide a good compromise between the above constraints,though, as mentioned above, a 4×4 array is usable. Also arrays havinghorizontal and vertical extents of 4 and 8, and 8 and 4, respectively,are usable, the latter being considered promising. At the very least, itis highly preferable for the number of ac luminance sub-bands to exceedthe number of dc luminance and chrominance sub-bands.

As an alternative to ignoring limited overspill or increasing the sizeof the sub-bands to reduce or remove overspill, it is possible to takeaccount of the fact that some chrominance information appears in bandsadjacent to these treated (in the quantization operation) as dcchrominance sub-bands by quantizing the adjacent sub-bands to an extentintermediate that to which they would be quantized if considered as acluminance sub-bands only, and that to which the sub-bands treated as dcchrominance sub-bands are quantized. The actual extent of quantizationof the adjacent sub-bands might well have to be established empirically.

As mentioned above, the use of a sampling frequency equal to four timesthe color sub-carrier frequency is preferred since it has the effect ofcentering the dc chrominance sub-bands about pi/2 in the horizontaldirection, that is locating them in the horizontal sense where shown inFIG. 14. However, other sampling frequencies can be used.

The foregoing description with reference to FIGS. 14 to 17 hasconcentrated on NTSC composite color video signals. It is to be noted,however, that the technique outlined above can be applied to otherbroadcast standard composite color video signals. The application of thetechnique to PAL composite color video signals will now be described.

FIG. 17 is a view corresponding to FIG. 4, but showing on thetwo-dimensional frequency plane both the sub-band filtered field of anNTSC composite color video signal, and a sub-band filtered field of aPAL composite color video signal, each sampled at four times its colorsub-carrier frequency. It will be seen that, in the case of PAL, thechrominance information occupies (in the case of an 8×8 array ofsub-bands) the four sub-bands (shown shaded) clustered at the center,rather than, as in the case of NTSC, the two at the bottom center,namely those numbered 24, 31, 32 and 39 in FIG. 10.

The only modification that has to be made to the apparatus as describedabove to enable it to handle a PAL color composite signal is to changethe numbers in the quantization matrix 52 that determine the amount ofquantization of the sub-bands that contain the chrominance data in thecase of PAL, namely the four PAL dc chrominance sub-bands as shownshaded in the center of FIG. 17. Specifically, instead of being heavilyquantized as high frequency ac luminance sub-bands of relatively littleimportance, the four sub-bands should be relatively lightly quantized soas to preserve the dc chrominance information. As in the case of NTSC,for PAL also the amount of quantization is in fact desirably reduced toabout the same level as applied to the dc luminance sub-band. Thenecessary effect can therefore be achieved by changing the four numbersclustered in the center of the quantization matrix as represented inFIG. 7 from their values of 260,396,396 and 581, for a component(luminance) signal, to 68 for a PAL composite signal. This is shownschematically in FIG. 7.

Further, in the case of PAL also, the apparatus is desirably furthermodified (as already indicated above) to change the zig-zag sequence oftreatment of the 63 sub-bands other than the dc luminance sub-band sothat the four dc chrominance sub-bands come first.

Since, in the case of PAL, the chrominance data occupies 4 of the 64sub-bands, whereas in the case of NTSC the chrominance data occupiesonly 2 of the 64 sub-bands, there is a slightly lower potential forcompression (as compared to NTSC) for PAL. Specifically, as indicatedabove, only 59/64 of a field in the case of PAL, as opposed to 61/64 ofa field in the case of NTSC, is occupied by ac luminance sub-bands andtherefore can be relatively heavily quantized on average.

Although the above-described apparatus operates on a field-by-fieldbasis, which will generally be more convenient, it could instead operateon a frame-by-frame basis. In this case the sub-bands would have twicethe number of samples in the vertical direction and the various fieldstores would be replaced by frame stores.

Further, although the above-described apparatus operates only on anintra-field basis, whereby sub-band filtering is effected in twodimensions or directions only, namely the horizontal and verticalspatial directions, it could in principle be extended to operate also onan inter-field or inter-frame basis, whereby sub-band filtering would inthis case be effected in three dimensions or directions, namely thehorizontal and vertical spatial directions and the temporal dimension ordirection.

Also, the apparatus described above may be embodied in an alternativemanner such that the sub-band data in the output store 28 of thesub-band filtering arrangement 12A or 12B is scanned in other ways thanthat described above with reference to FIGS. 9 to 11, according to whichthe dc sub-band is scanned first and the 63 ac sub-bands are thenzig-zag scanned in the sequence or order shown in FIG. 10 or, in thecase of a composite color signal, a modified version of that sequence inwhich the dc chrominance sub-bands come first. The sequence could forexample be changed so that the ac sub-bands are scanned in a series ofstraight lines rather than in a series of zig-zag diagonal lines.Alternatively, as is described in our copending U.S. patent applicationSer. No. 07/810,336, which is assigned to the assignees hereof, whichwas filed on the same day as the present application, and whichcorresponds to UK Patent Application No. 9100593.4 filed Jan. 11, 1991,instead of first scanning the dc sub-band and then scanning the 63 acsub-bands in a zig-zag sequence it is possible to scan all the 64sub-bands in zig-zag (or other) sequence.

According to another alternative manner of embodying the apparatusdescribed above, the entropy encoder 16A of FIG. 12 is modified byeliminating the DPCM 64, the data modeller 72, the VLC PROM 74 and theFLC PROM 76, and connecting the output of the switch 60, that formerlywas connected to the DPCM 64, directly to the multiplexer 70. In thisevent, whereas all the quantized data derived from the ac sub-bands areentropy encoded, the data derived from the dc sub-band are not entropyencoded, which in some cases may be advantageous. Naturally, thisresults in a reduction in overall signal compression, though thereduction is minor since the dc sub-band is of course only one of manysub-bands whereby the bulk of the quantized data is entropy encoded.

FIG. 18 is a block circuit diagram of a video signal storage apparatusembodying the invention. The video signal storage apparatus of FIG. 18comprises a video signal compression apparatus or means as describedabove, namely the sub-band filtering arrangement 12A described abovewith reference to FIGS. 2 and 3 or the sub-band filtering arrangement12B described above with reference to FIG. 5, the quantizer 14Adescribed above reference to FIG. 8, and the entropy encoder 16Adescribed above reference to FIG. 12. The video signal compressionapparatus or means may include a field buffer 90 which, as shown, may beconnected to the output of the entropy encoder 16A, though it couldinstead be connected between the quantizer 14A and the entropy encoder16A. A signal indicative of the content (state of fullness) of the fieldbuffer 90 may be fed by a line 92 back to the scale factor generator 56(FIG. 8) of the quantizer 14A so as to control the scale factor in sucha manner that the amount of data per field after compression neverexceeds a predetermined value and, on the other hand, never fallsgreatly below the predetermined value. Thus, each compressed field canbe recorded in or on a predetermined part of a storage medium.

The video signal compression apparatus or means is followed by a storagemeans 94 for storing successive fields or frames of the compressed videosignal. The storage means 94 may take a variety of forms. It may, forexample, comprise a random access memory (RAM) recorder, that is anamount of RAM sufficient to store many fields or frames. It may insteadcomprise a disk recorder, that is an arrangement for writing thecompressed signal, for example magneto-optically, to a magnetic storagemedium in the form of a magnetic disk. Further, the storage means 94 mayinstead comprise a video tape recorder (VTR), that is to say anarrangement for writing the compressed signal to a magnetic storagemedium in the form of a magnetic tape.

FIG. 19 shows one way in which the storage means 94 may be implementedin the form of a VTR. The storage means 94 of FIG. 19 comprises ablocking circuit 96 which divides compressed data read from the fieldbuffer 90 into segments of fixed length and selectively supplies them toone of two error correction encoders 98A and 98B. Specifically, by wayof an example, the compressed data resulting from quantization andentropy encoding of the dc luminance sub-band is directed to the errorcorrection encoder 98A, while the compressed data resulting fromquantization and entropy encoding of the other sub-bands is directed tothe error correction encoder 98B. The encoder 98A applies a more robust(more highly protective) error correction code than the encoder 98B,whereby the data derived by compression from the dc luminance sub-bandis subjected to more robust error correction than the data derived bycompression from the other sub-bands. This feature provides that, ingeneral in the case of reproduction, the dc sub-band (of most importanceto the human psychovisual system) is given special attention from thestandpoint of error correction so as to minimize degradation of picturequality as a result of compression and storage, and subsequentreproduction and expansion. Further, in the specific case of reproducingand expanding data in a shuttle (high speed reproduction) mode of theVTR, this feature provides the further advantage that the probability ofsuccessful data recovery is enhanced.

The error correction encoded data emerging from the encoders 98A and 98Bis recombined and directed by a switch circuit 100 to a formattingcircuit 102 in which that data is arranged, together with (at least)audio data and synchronization data, into a format to be recorded ontape. The operation performed by the formatting circuit 102 is thusconventional, though (as explained below) the actual format employed ispreferably not conventional.

Formatted data outputted by the formatting circuit 102 is, inconventional manner, directed via a channel coder 104 and a radiofrequency (RF) circuit 106, containing a modulator and so forth, to arecording head 108 which is mounted on a drum and is moved in use withrespect to a tape in contact with the drum so as to record the formatteddata in slanting tracks on the tape.

By appropriate design of the formatting circuit 102, the data derived bycompression from the dc luminance sub-band can be recorded on tape in adifferent manner than the data derived by compression from the othersub-bands. This leads to the advantage that the dc luminance sub-bandcan be recorded so as to improve the probability of data recovery inshuttle.

The dc sub-band information can thus, for example, be recorded atpredetermined positions along the tracks on the tape, for example halfway along them as shown in FIG. 20, in which a slanting track 120recorded on a tape 122 has the dc sub-band information recorded at aportion 124 thereof shown shaded. In this regard, when the VTR is in theshuttle mode a reproducing head thereof is generally unable (exceptperhaps if it is a dynamic tracking head and the shuttle speed is lessthan about two or three times the normal reproduction speed) to followthe tracks. Thus, the locus of travel (shown at 126 in FIG. 20) of thereproducing head in the shuttle mode is skewed with respect to thetracks and therefore crosses the tracks rather than follows them. Thus,information is recovered only where the crossings take place, ratherthan along the whole lengths of the tracks. It is a matter of relativeease to synchronize the movement of the reproducing head with the tracksso that the locus of travel of the head always crosses the tracks at thepredetermined positions at which the dc luminance sub-bands ofsuccessive fields are recorded. Thus, while recovery of data from otherparts of the tracks may be difficult or impossible to achieve in theshuttle mode, recovery of the dc luminance sub-bands should beachievable with relative ease. An advantage of being readily able torecover at least the dc luminance sub-bands of successive fields isthat, as explained above, if that sub-band alone is reproduced, expandedand displayed it is visually intelligible in that it looks like a veryheavily filtered version of the original picture. Thus, the position ontape can be visually monitored even at very high shuttle speeds byrecovering and viewing only the dc luminance sub-bands of successivefields (possibly together with one or more of the ac luminance sub-bandsof lower spatial frequency information content).

An additional or alternative technique of recording the data derived bycompression from the dc luminance sub-band on tape in a different mannerthan the data derived by compression from the other sub-bands, leadingto the advantage that the dc luminance sub-band can be recorded so as toimprove the probability of data recovery in shuttle, comprises recordingthe data derived by compression from the dc luminance sub-band at two ormore places on the tracks: see, for example, FIG. 21, where the dcluminance sub-band data is recorded at two portions (shown shaded) 128a,128b of the track 120. This can readily be achieved by appropriatedesign of the formatting circuit 102.

The apparatus described above by way of example with reference to FIGS.18 and 19 can be modified in a variety of ways. For instance, althoughin the storage means 94 of FIG. 19 the data derived by compression fromthe dc luminance sub-band is subjected to one form or level of errorcorrection (in the encoder 98A) and all the data derived by compressionfrom the other sub-bands is subjected to another, common form or levelof error correction (in the encoder 98B), a more highly selective formof error correction encoding is possible. In general, each sub-band canbe subjected singly, or as part of a group of sub-bands, to its ownrespective form or level of error correction encoding: this mightnecessitate redesign of the sequence of quantizing and/or redesign ofthe entropy encoder 16A so that the data resulting from compression ofthe sub-bands other than the dc luminance sub-band is readilydistinguishable at the output of the entropy encoder. (In this regard,with the arrangement specifically described above, the scanningtechnique described with reference to FIGS. 9 to 11 results in the acluminance sub-bands being intermingled before entropy encoding so thatthe portion of the compressed signal derived from the ac luminancesub-bands cannot readily be divided, after entropy encoding, into partsderived from the different ac luminance sub-bands. However, if, forexample, the technique shown in FIG. 9 were modified so that at leastsome of the sub-bands other than the dc luminance sub-band were scannedin the same way as the dc luminance sub-band, and if those sub-bandswere entropy encoded in respective different ways, then the compressedversions of those sub-bands would be readily distinguishable afterentropy encoding and could therefore readily be treated differently toone another as regards error correction encoding.)

Preferably, in the case when a composite color video signal is beingcompressed and stored, the data derived by compression of the dcchrominance sub-bands (as well as the data derived by compression of thedc luminance sub-band) is, due to its relative importance, subjected tomore robust error correction encoding than the data derived bycompression of the other sub-bands.

Further, in the case of either a component or composite signal, it wouldbe desirable, in view of the relative importance of the information theycontain, also to apply more robust error correction encoding to thecompressed data obtained from the first few ac luminance sub-bands, thatis those of relatively low spatial frequency information content.

The form of implementation of the storage means 94 described above withreference to FIG. 19 can be used, with appropriate changes, forrecording on disk or in RAM. While disk storage is in general less errorprone than tape storage, the selective or differential error encodingmay at least in some cases be desirable in the case of disk storage.And, while RAM storage is in general even less error prone, theselective or differential error encoding might in some cases be usefuleven in the case of RAM storage. A form of formatting similar to thatprovided in the case of tape storage may be desirable in the case ofdisk storage, but would be unnecessary in the case of RAM storage.

The apparatus as described above with reference to FIGS. 18 and 19assumes that only one recording head 108 is needed to record thecompressed signal on tape. While a single recording head may besufficient in some cases, plural heads may have to be used if the datarate is too high for the data to be handled by the limited bandwidth ofone head. In that case, the data could be demultiplexed (for example ona line-by-line or sample-by-sample basis)before or after the entropyencoder 16A and the plural demultiplexed data streams each passed to arespective storage means 94 as described with reference to FIG. 19. Ifthe demultiplexing took place before the entropy encoder 16A, theentropy encoder (as well as the storage means 94) would need to bereplicated so that there was one for each demultiplexed data stream.

Although illustrative embodiments of the invention have been describedin detail herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various changes and modifications can be effectedtherein by one skilled in the art without departing from the scope andspirit of the invention as defined by the appended claims.

We claim:
 1. Apparatus for storing a digital video signal, the apparatuscomprising:signal compression means including a spatial two-dimensionalsub-band filtering arrangement for filtering said digital video signalto form a plurality of data sets constituting respective sub-bands ofthe two-dimensional spatial frequency domain, a quantizer for quantizingsaid data sets in accordance with respective values, said values beingsuch that the amount of quantization of one of said data setsconstituting a sub-band to which dc luminance information of saiddigital video signal is at least predominantly confined is less than theaverage of the amounts of quantization of the remaining data sets, andan entropy encoder for selectively encoding the quantized data sets sothat said quantized data sets as selectively entropy encoded constitutea compressed video signal; error correction encoding means for providingdifferent error correction encoding as between different portions ofsaid compressed video signal each derived from a respective one or moreof said data sets so as to form an error correction encoded videosignal; and storage means for storing said error correction encodedvideo signal.
 2. Apparatus according to claim 1, wherein the storagemeans comprises magnetic tape storage means.
 3. Apparatus according toclaim 1, wherein the storage means comprises magnetic disk storagemeans.
 4. Apparatus according to claim 1, wherein the storage meanscomprises random access memory storage means.
 5. Apparatus according toclaim 1, wherein said storage means is operative to record thecompressed video signal on a magnetic storage medium, and storagecontrol means is provided to cause a portion of the compressed signalderived from said data set constituting said sub-band to which the dcluminance information of the signal is at least predominantly confinedto be recorded on said storage medium differently than the remainder ofthe compressed signal.
 6. Apparatus according to claim 1, wherein saidstorage means is operative to record the compressed video signal inslanting tracks on a magnetic tape, and storage control means isprovided to cause a portion of the compressed signal derived from saiddata set constituting said sub-band to which the dc luminanceinformation of the signal is at least predominantly confined to berecorded on said tracks differently than the remainder of the compressedsignal.
 7. Apparatus according to claim 6, wherein said storage controlmeans is operative to cause said portion of the compressed signalderived from said data set constituting said sub-band to which the dcluminance information of the signal is at least predominantly confinedto be recorded at predetermined positions along at least some of saidtracks.
 8. Apparatus according to claim 7, wherein said predeterminedpositions are at the centers of at least some of said tracks. 9.Apparatus for storing a digital video signal, the apparatuscomprising:signal compression means including a spatial two-dimensionalsub-band filtering arrangement for filtering said digital video signalto form a plurality of data sets constituting respective sub-bands ofthe two-dimensional spatial frequency domain, a quantizer for quantizingsaid data sets in accordance with respective values, said values beingsuch that the amount of quantization of one of said data setsconstituting a sub-band to which dc luminance information of saiddigital video signal is at least predominantly confined is less than theaverage of the amounts of quantization of the remaining data sets, andan entropy encoder for selectively encoding at least some of thequantized data sets so that said quantized data sets as selectivelyentropy encoded constitute a compressed video signal; error correctionencoding means for providing different error correction encoding ofdifferent portions of the compressed video signal each derived from arespective one or more of said data sets so as to form an errorcorrection encoded video signal with more robust error correctionencoding being provided to a portion of the compressed video signalderived from said data set constituting said sub-band to which the dcluminance information of the signal is at least predominantly confinedthan to at least part of the remainder of the signal; and storage meansfor storing said error correction encoded video signal.
 10. Apparatusaccording to claim 9, wherein, in order to enable compression of adigital composite color signal, said quantizer is operative to quantizesaid data sets in accordance with respective values which are such thatthe amounts of quantization of each of said one of said data setsconstituting said sub-band to which the dc luminance information of thesignal is at least predominantly confined and of at least two of saiddata sets constituting sub-bands to which dc chrominance information ofthe signal is at least predominantly confined are less than the averageof the amounts of quantization of the other data sets, and wherein saiderror correction coding means is operative to provide more robust errorcorrection encoding to portions of the signal derived from said data setconstituting said sub-band to which the dc luminance information of thesignal is at least predominantly confined and from said data setsconstituting said sub-bands to which the dc chrominance information ofthe signal is at least predominantly confined than to the remainder ofthe signal.
 11. Apparatus according to claim 10, wherein said quantizeris so operative that the amounts of quantization of said sub-bandsconstituted by the data sets to which the dc de luminance informationand the dc chrominance information is at least predominantly confinedare at least approximately the same as one another.
 12. Apparatus forstoring a digital video signal, the apparatus comprising:signalcompression means including a spatial two-dimensional sub-band filteringarrangement for filtering said digital video signal to form a pluralityof data sets constituting respective sub-bands of the two-dimensionalspatial frequency domain, a quantizer for quantizing said data sets inaccordance with respective values, said values being such that theamount of quantization of one of said data sets constituting a sub-bandto which dc luminance information of said digital video signal is atleast predominantly confined is less than the average of the amounts ofquantization of the remaining data sets, and an entropy encoder forselectively encoding at least some of the quantized data sets so thatsaid quantized data sets as selectively entropy encoded constitute acompressed video signal; storage means for recording said compressedvideo signal on a magnetic storage medium; and storage control means forcausing a portion of the compressed video signal derived from said dataset constituting said sub-band to which the dc luminance information ofsaid digital video signal is at least predominantly confined to berecorded on said magnetic storage medium at least twice.
 13. Apparatusfor storing a digital video signal, the apparatus comprising:signalcompression means including a spatial two-dimensional sub-band filteringarrangement for filtering said digital video signal to form a pluralityof data sets constituting respective sub-bands of the two-dimensionalspatial frequency domain, a quantizer for quantizing said data sets inaccordance with respective values which are such that the amounts ofquantization of each of said one of said data sets constituting saidsub-band to which the dc luminance information of said digital videosignal is at least predominantly confined and of at least two of saiddata sets constituting sub-bands to which dc chrominance information ofthe signal is at least predominantly confined are less than the averageof the amounts of quantization of the other data sets, and an entropyencoder operative to selectively encode at least some of the quantizeddata sets; so that said quantized data sets as selectively entropyencoded constitute a compressed video signal; and storage means forstoring the compressed video signal.
 14. Apparatus according to claim13, wherein said quantizer is so operative that the amounts ofquantization of said sub-bands constituted by the data sets to which thedc luminance information and the dc chrominance information is at leastpredominantly confined are at least approximately the same as oneanother.